1. Field of the Invention
This invention relates to an automatic exposure time control circuit of digital design for use with a single lens reflex camera having a through-the-lens-type light measuring system.
2. Description of the Prior Art
As is well known in the art, a common type of automatic exposure time control circuit for a single lens reflex camera of the through-the-lens light measuring type responds to the scene brightness and stores information representative of the sensed level of the brightness, prior to actuation of the camera shutter. The exposure time thus is controlled as a function of the stored information. Typically, a voltage is generated which is proportional to the logarithm of the scene brightness (referred to as a logarithmic compression) and the voltage is stored, for example, as an analog value in a capacitor or, after A-D conversion, is stored in the form of a digital voltage. Closing of the shutter, and thus control of exposure, is performed in relation to the stored value of the scene brightness (this operation typically being referred to as logarithmic expansion).
In the known circuits as described above, in the case of exposure, the brightness information converted into a digital voltage, is reconverted into an analog current proportional to the scene brightness, by which a capacitor is charged, and the time for the charging of the capacitor to reach a predetermined level is used to control the exposure time. In this case, a diode or transistor having a logarithmic characteristic is used, with the result that a temperature compensating circuit is required to provide accuracy. Adjustment of the circuit, however, is time-consuming. Further, the A-D conversion of the voltage proportional to the logarithm of the scene brightness, mentioned above, inevitably results in a conversion error between the analog value and the digital value.